input:map.in output:map.out

时间限制: 1000 ms 空间限制: 262144 KB

题目描述

输入

输出

样例输入

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2 1 1 2

样例输出

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2

数据范围限制

代码

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#include<bits/stdc++.h>
#define ll long long
#define maxn 2010
#define mod 998244353
using namespace std;
ll f[maxn][maxn];
int main()
{
    freopen("map.in", "r", stdin);
    freopen("map.out", "w", stdout);
    int n, s1 = 0, s2 = 0;
    scanf("%d", &n);
    for (int i = 1; i <= n; i++)
    {
        int x;
        scanf("%d", &x);
        if (x == 1) s1++;
        else s2++;
    }
    f[0][0] = 1;
    for (int tot = 0; tot <= n; tot++)
        for (int i = tot; i >= 0; i--)
        {
            int j = tot - i;
            if (j < 0) continue;
            if (j == 0 && i) f[i + 1][j] = (f[i + 1][j] + f[i - 1][j] * i % mod) % mod;
            if (i >= 2)
            {
                ll tmp = f[i - 2][j] * ((ll)i * (i - 1) / 2 % mod) % mod;
                f[i][j + 1] = (f[i][j + 1] + tmp) % mod;
            }
            if (j >= 2)
            {
                ll tmp = f[i + 2][j - 2] * ((ll)j * (j - 1) / 2 % mod) % mod;
                f[i][j + 1] = (f[i][j + 1] + tmp) % mod;
            }
            if (i && j)
            {
                ll tmp = f[i][j - 1] * i % mod * j % mod;
                f[i][j + 1] = (f[i][j + 1] + tmp) % mod;
            }
        }
    printf("%lld\n", f[s1][s2]);
    return 0;
}